If the dead keys are in random places, then I suspect a spillage, if all in one row, then maybe an electrical fault.
Remove the keycaps and take a look at the membrane. If there has been a large spillage, then it may be difficult to repair, especially if sugar (coffee, Coke) has been involved. I had a G4 A1139 with random dead keys. I stripped the keyboard and gave the filthy membrane the best clean I could before reassembling. After three attempts I got all but the backspace key working. Nothing would fix it. I replaced the keyboard. Luckily for me, this was a couple of years ago when keyboards were plentiful and not too expensive.
On another G4 PB, the entire top row didn't work, so no Fn keys or shortcuts. I couldn't see how to fix it as nothing was obviously broken - no broken traces on the membrane or broken ribbon cables. The keyboard connector socket on the logic board seemed ok, too. I cleared NV strings etc just in case it was some weird OF voodoo but nothing fixed it. I just live with that one.
